MIXED CHOP OF UNIFOBM AGE. 49 



be able, otber tbings being equal, to resist botb frost and drought, 

 and, therefore, the more likely is that species to predominate over less 

 favoured associates. Instances in point are such gregarious species 

 as sal, Hardwickia, &c. The deeper situated any layer of the 

 soil is, the larger will be the quantity of moisture in it and the 

 less exposed will it be to wide fluctuations of temperature due to 

 subaerial variations. Hence the deeper a taproot goes down, the 

 more likely is it to find sufficient moisture, and the higher will be 

 the temperature of the moisture passing through it into the plant- 

 portion above during frosty weather the one circumstance tend- 

 ing to protect the plant against drought, the other against frost- 

 bite. Thus, in spite of their extreme sensitiveness to frost, young 

 Terminalia tomentosa are often able to survive in low open frosty 

 situations, even though they may never be able to grow up owing 

 to the annual shoot being killed down each winter. A long tap- 

 root gives yet another advantage. Since, during heavy frost, the 

 water contained in a certain thickness of the soil (which thickness 

 will of course vary with the severity of the frost and the character 

 of the soil) freezes and expands, causing the soil 'to swell up with 

 itself, seedlings with a short taproot, after rising with the soil as 

 this latter expands, must inevitably be left ejected when, on thaw 

 occurring, the now soft and muddy soil settles down again. On 

 the other hand, seedlings with a long and strong taproot would be 

 generally safe against such a catastrophe, unless the taproot were 

 torn asunder by the uprising of the superficial layers of the soil. 

 The behaviour of individuals of one and the same species in res- 

 pect of each other, as their root-apparatus goes on expanding with 

 the progressive growth of the crop, is of course the same here as in 

 the First Case, except in so far as it is modified by the action of 

 other spe-cies present in the crop. This action has now been ex- 

 plained and will be still better understood after a persual of the 

 remarks made under this head in the Second Case. It is only after 

 bringing together all the various circumstances detailed to a com- 

 mon focus that we can adequately appreciate the importance of the 

 role played in the struggle for existence by a difference of root- 

 systems. That role is not inferior to the part appertaining to a 

 difference of crown-development and of requirements as to light 

 and shelter. No single species can utilise all the space in the soil 

 any more than it can utilise all the space in the leaf-canopy above; 

 and just as two species of different habits can grow up together 

 side by side, whereas two others having a similar habit cannot 

 without mutual injury resulting, so within a given area the soil may 



